Based on the promo released for episode 2x06 of Shadowhunters.
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"Come here and kiss me, Alexander," Magnus said with confidence and a raising of his eyebrows. Alec smiled and did as he said. He took a step forward as Magnus' lips curled into a smile of his own. Alec grabbed Magnus by the waist and pulled him into a smiling kiss. Magnus wrapped his arms around Alec's neck and smiled back into the kiss.
Alec quickly pinned Magnus to the nearest wall. He grinned down at Magnus, though he didn't say anything. Magnus chuckled.
"Nice try, Alexander...," he started. Suddenly, Magnus switched places with Alec and held his wrists up against the wall, "...but I've been alive for 400 years. When you live for that long, you gain some," he flashed his cat eyes up at Alec, "experience."
Alec's eyes widened slightly at the warlock's remark, and he had to take a moment to breathe properly. He blushed and smiled at the same time.
"Then, uh...why don't you show me what you've learned?" Magnus smirked and tightened his grip on Alec's wrists as he kissed him again. He pulled away from Alec's face.
"I'd love to." Magnus pulled Alec off of the wall and led him while facing backwards to the couch. He laid himself down on it, still holding onto Alec's wrists, and yanked Alec on top of him. As they were making their way onto the couch, they kissed sloppily but lovingly, and Magnus let go of Alec to return his arms to his neck. Magnus wrapped his legs around Alec, while Alec had his elbows on either side of Magnus' head.
The two continued passionately until a loud noise coming from the foyer made them stop. They looked up dazedly to find Simon turned the other way with one hand rubbing the back of his head, the other shoved into his jacket pocket. What they had heard was Simon yelping after he had accidentally walked in on them.
Simon turned around slightly and slowly, for fear of what he would see if he turned around completely. Not gaining any sort of response, he turned all the way around. Magnus and Alec didn't move from where they were. They exchanged a glance and looked back at Simon. Magnus spoke first.
"Can we help you?" he said in a slightly annoyed tone. Alec added on.
"Yeah, we're kind of in the middle of something, vampire." Magnus ignored Alec's snide use of "vampire" and continued staring at Simon. The situation was oddly amusing, since usually when a couple gets walked in on, they're the ones who are supposed to feel embarrassed, not the person who caught them. Simon moved his mouth to say something, but the words wouldn't form. Alec raised his eyebrow, waiting for an answer. Finally, he spoke.
"I...um...I'll just uh, I'll come back l-later or something, uh...t-tex-text me when you're not busy again, M-Magnus...," said Simon, obvious discomfort in his voice and body language. He stalked out of the room, too flustered to use vampire speed.
Magnus wondered for a moment why Simon needed to see him to begin with, but he shrugged it off. He had more important things to attend to right now.
Turns out Simon needed help with evading his family, and Magnus partially wished he had stopped him and Alec earlier since it's such an important matter.
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Though this was fun to write, I'm pretty sure the initial question was something along the lines of "Where do you want to go for our date, Alexander?"
